iOS painting and drawing library based on Metal. 神笔马良有一支神笔（基于 Metal 的涂鸦绘图库）
fix color space issue #104
fix logic about opacity calculation. So the opacity of actually painted line will near to the value of brush opacity and current color's alpha value.
This update may affect your original brush's function, be careful to update.
set 'isPencilMode' of Canvas to 'true' to enable pencil only mode on iPad with Apple Pencil
fix an issue that canvas not clear when function clear() is called
refers to issue #73